The interaction of human glycophorin with 8-anilino-1-naphthalene sulfonate

Both the sialoglycoprotein of human erythrocyte membranes, glycophorin, and the sialic acid free protein, obtained by treatment of glycophorin with neuraminidase (EC 3.2.1.18), increase the fluorescence of 8-anilino-1-naphthalene sulfonate (ANS). Binding of ANS to glycophorin is weak compared with the binding to bovine serum albumin (BSA). equilibrium dialysis gives an apparent binding constant of about 4 X 10(3) M(-1) at neutral pH, but Ka increases 1.75 times when NaCl or CaCl2 are added and 10-fold when the pH is lowered to 3.0. Sialic acid groups do not significantly affect ANS binding, although they have some effect at low ionic strength and neutral pH. Fluorescence studies indicate only one to two binding sites for ANS, with apparent pK = 3.8 +/- 0.2, and located close to aromatic residues in glycophorin. Polarization and quantum efficiency of the fluorescence of ANS associated with glycophorin fail to indicate changes in the vicinity of the binding site when the pH is lowered.